Tony't right, beware of using debit cards. There's always a heavy charge by your debit card provider for using their cards abroad, even in this country you can pay commission to buy currency if using your debit card.
The exchange rate in SS (after commission) was 1.17 yesterday, this was confirmed to me by Noula (San Stefanos Travel) so there is not a huge amount of difference between GB and San Stefanos.
My advice on earlier postings was to take one of the "no commission credit cards" ie Nationwide, Post Office etc, but beware that some restaurants/tavernas will try to charge you 2% commission, even so it's better than using a debit card and can still be better than paying by cash.
Travellers cheques are still a good idea, if you lose them you can have them replaced at short notice, with cash, once it's lost it's gone forever.